1. Field of the Invention
The present invention relates to a high frequency filter mainly used in a VHF band, UHF band, a microwave band and a millimeter wave band and more particularly to polarize it and improve its characteristic.
2. Description of the Related Art
FIG. 33 is a schematic diagram showing a conventional high frequency filter disclosed in e.g. Japanese Utility Model Unexamined Publication No. Hei 1-101603.
In the figure, reference numeral 8c denotes a dielectric block.
Reference numeral 9 denotes one of outer conductors each made of a conductive film formed on the remaining sides other than one side (upper side in the figure) of all the sides of the dielectric block 8c. The outer conductors are in intimate contact with the outer wall of the dielectric block 8c.
Reference numeral 10c denotes one of inner conductors each made of a conductive film formed in intimate contact with the inner wall of each of first through-holes 23 described later. The inner conductors 10c are continuously connected to the outer conductors 9 on the outer wall of the dielectric block 8c at their one ends (the bottom side in the figure).
Reference numeral 23 denotes one of first four through-holes passing through between opposite faces of the dielectric block 8c (upper face and bottom face) and arranged in substantially parallel. The first through-holes 23 are arranged in substantially parallel to the remaining faces (side faces in the figure).
Reference numeral 24 denotes one of three second through-holes formed in the same manner as the first through-holes 23 and arranged in substantially parallel between the adjacent first through-holes 23. Each second through-hole 24 has a smaller diameter than that of each first through-hole 23.
The inner conductors 10c, first through-holes 23 and second through-holes 24 constitute 1/4 wavelength resonators 120a to 120d with their one ends opened and other ends short-circuited.
Reference numeral 25 denotes one of first electrodes formed on the surface of the dielectric block 8c at the open ends (upper side in the figure) of the 1/4 wavelength resonators 120a and 120d at both ends. Each first electrode is continuously connected to each inner conductor 10c.
Reference numeral 26 denotes a dielectric plate having substantially the same shape as the one side (upper face in the figure) of the dielectric block 8c. The dielectric plate 26 is overlaid on this surface of the dielectric block 8c.
Reference numeral 27 denotes one of third through-holes provided on the dielectric plate 26 so as to coincide with the opening positions of the first through-holes 23 at the open ends of the 1/4 wavelength resonators 120a and 120b at both ends.
Reference 28 denotes one of second electrodes each made of a conductive film in intimate contact with the surface of the dielectric plate 26 and formed on the periphery of each of the second though-hole at both ends.
Reference numeral 29 denotes a conductor for connecting said second electrodes 28 to each other.
Reference numeral 30 denotes a dielectric tube, and P1 and P2 denote terminals provided on the dielectric tube 30, respectively.
Each first electrode 25 and each second electrode 28 are opposite to each other through the dielectric plate 26 to constitute a capacitor. The terminal P1 and the terminal P2 are partially inserted into dielectric tubes 30, respectively and into the through-holes 23 at both ends. Thus, the inner conductor 10c, dielectric tube 30, terminal P1 or P2 constitute a capacitor for input/output coupling.
An explanation will be given of the operation theory. First, the presence of the second through-holes 24 generates inequality in permittivity within the dielectric block 8c. This inductively couples the adjacent resonators to each other by mainly a magnetic field. The amount of coupling can be adjusted by the distance between the resonators 120 and size of the second through-hole 24. The resonators 120a and 120d at both ends are mainly inductively coupled with each other through the intermediate resonators 120b and 120c, and also slightly capacitively coupled with each other through the first electrodes 25, second electrode 27 and connecting conductor 29.
Now it is assumed that the length of the inner conductor 10c is adjusted so that the four resonators 120a to 120d are resonated at the same frequency f0. On this assumption, at the frequency f0, the four resonators in a resonance state are strongly inductively coupled with one another. Thus, a wave incident to the terminal P1 is guided to the resonator 120d through the resonators 120a to 120c and taken out from the terminal P2. On the other hand, at the frequency other than f0, the resonators 120a to 120d are very weakly coupled with one another so that most of the electric power of the incident wave to the input/output terminals is reflected. In this way, the conventional high frequency filter as shown in FIG. 33 can serve as a band-pass filter.
In the high frequency filter as shown in FIG. 33, the resonators 120a and 120d at both ends are mainly coupled with each other through the intermediate resonators 120b and 120c and also slightly capacitively jumping-coupled with each other by the first electrodes 25, second electrodes 27 and connecting conductor 29. Generally, the passing phase of the resonator is +90.degree. at the frequency lower than the resonance frequency, 0.degree. at the resonance frequency and -90.degree. at the frequency higher than the resonance frequency. The passing phase of the capacitive coupling means in series connection is +90.degree. whereas that of the inductive coupling means in series connection is -90.degree.. In the main coupling between the resonators 120a and 120d at both ends, which passes through two resonators and three stages of inductive coupling means, the total passing phase is -90.degree. at the frequency lower than f0 and -450.degree. (=-90.degree.) at the frequency higher than f0.
On the other hand, since the jumping-coupling is capacitive, the passing phase due to it is +90.degree. irrespectively of the frequency. Thus, in the conventional high frequency filter as shown in FIG. 33, the passing phase due to the main coupling and that due to the jumping-coupling are opposite. For this reason, attenuation poles are generated in the frequencies lower and higher than the passing band, thereby making the attenuation characteristic abrupt. In this case, the amount of jumping which is very little has little effect on the loss of the passing band.
In order that the jumping-coupling is capacitive, it should be noted that the electric length of a connecting conductor must be much shorter than the wavelength, and in FIG. 33, the permittivity of the dielectric plate 26 must be much smaller than that of the dielectric block 8c.
FIG. 34 is a schematic diagram showing the conventional high frequency filter disclosed in J-UM-3-44304, for example.
In the figure, reference numeral 8 denotes a dielectric plate.
Reference numeral 9 denotes an outer conductor of a conductive film formed in intimate contact with the one entire surface (bottom surface in the figure) of the dielectric plate 8.
Reference numeral 10 denotes one of strip conductors of a conductor film arranged in parallel and formed in intimate contact with the other surface (upper surface in the figure) of the dielectric plate.
Reference numeral 11 denotes a short-circuiting end surface of a conductive film formed in intimate contact with the side surface of the dielectric plate and continuously connected to the outer conductor 9 and strip conductors 10.
The dielectric plate 8, outer conductor 9, strip conductors 10 and short-circuiting end surface 11 constitute an approximately 1/4 wavelength microstrip line type resonator 110 with the one end opened and other end short-circuited.
Reference numeral 13 denotes one of capacitors provided on the strip conductors 10, respectively.
Reference numeral 14 denotes one of conductor ribbons each having the one end connected to the capacitor 13 and the other end connected to a strip conductor 31 described below.
Reference numeral 31 denotes the strip conductor of a conductor film in intimate contact with the other surface (upper surface in the figure) of the dielectric plate 8. The strip conductor 31 is arranged in a direction crossing the strip conductors 10 in the vicinity of the open ends of the strip conductors where the capacitors 13 are provided.
The dielectric plate 8, outer conductor 9 and strip conductor 31 constitute a main line 32.
Reference symbols P1 and P2 denote terminals, respectively. The open ends of the two strip conductors are connected to the strip conductor 31, with a distance of approximately 1/4 wavelength therebetween, through the capacitors 13 and conductor ribbons 14.
In operation, assuming that the resonance frequency of the resonator 110 is f0, at the frequency lower than f0, the resonator 110 serves as an inductance to constitute a series resonance circuit together with a capacitor 13. Now assuming that the series resonance frequency is f1, most of the electric power of the incident wave at the frequency of f1 incident on the terminal P1 is reflected owing to the resonance in the series resonance circuit. On the other hand, at the frequency other than f1, without being influenced by the resonators 110, most of the electric power of the incident wave on the terminal P1 is guided to the terminal P2. In this way, the conventional high frequency filter as shown in FIG. 34 serves as a band-stop filter.
Since the high frequency filter is constructed as described, where the resonators 120a to 120d and the jumping-coupling means such as the electrodes 25 and 27 are formed on the same dielectric block or plate, or otherwise the permittivity of the dielectric material constituting a filter is relatively small, the electric length of the connection line (connection line 29) of the jumping connecting means becomes fairly long, thus making it impossible to form a desired attenuation pole.
In connection between the strip conductors 10 and strip conductor 31, in addition to the connection in the manner of a lumped constant circuit by the capacitors 13, the direct connection by fringing is provided so that both strip conductors cannot be arranged adjacently to each other. For this reason, the conductor ribbons are required for connecting the capacitors 13 to the strip conductors 31. This makes the assembling of the high frequency filter complicated.